Wireless communication technology has been researched and developed to increase the amount of transmission data and the data transmission rate via user equipment (UE) or eNBs. In order to perform communication between UE and an eNB, they need to be synchronized with each other and thus can transmit/receive data to/from each other. In this case, a link or channel for communication from UE to an eNB is called “uplink,” and a link or channel for communication from an eNB to UE is called “downlink.”
In order to obtain synchronization between UE and an eNB in a wireless communication system, UE is capable of obtaining time synchronization via a Synchronization CHannel (SCH) as a downlink from an eNB via. In this case, the synchronization information that UE obtained via SCH receives data in the downlink and may be synchronization information required to transmit data in the uplink. If UE needs to perform data communication with an eNB, it may request access from the eNB via an uplink Random Access CHannel (RACH). When UE performs an access procedure to an eNB via an RACH, it sequentially increases uplink power and performs the transmission of an RACH preamble. If UE transmits an RACH preamble to the eNB with an appropriate level of power, the eNB may receive the RACH preamble. If the eNB receives an RACH preamble, it may create an RACH Response (RAR) message and transmit the RAR message to the UE. Therefore, the UE is capable of ascertaining a level of power which is suitable for the uplink transmission. In addition, the operation described above may also allow the eNB to set up a transmission time difference or Timing Advanced (TA) between the UE and the eNB as well as a level of power for the uplink transmission.
In recent years, a beamforming technique as a system for transmitting a relatively large amount of data at high rate has been used for various wireless communication technologies. In a state where both UE and an eNB employ a beamforming technique, if a procedure for accessing an eNB via an RACH is performed, an RACH preamble transmission needs to be performed according to transmission beams and reception beams. That is, if an eNB and UE, employing a beamforming technique, need to perform an RACH procedure, they need to scan and measure all beams, and determine the best uplink transmission beam and the best reception beam among the measured beams.
In a state where both UE and an eNB employ a beamforming technique, if the UE transmits, to the eNB, an RACH preamble for the initial access, it needs to increase the number of transmission beams and reception beams and thus increases the burden of control to measure and determine a best beam for the beamforming. In addition, the increase in the number of transmission beams and reception beams causes time delay in determining the best transmission beam and reception beam. In particular, in an environment where a macro-cell and a small cell are overlapped, the increase in the initial access time to every eNB and the burden of control causes a corresponding cell to increase its load and also to transmit a relatively large amount of RACH preambles, causing interference with other channels.